So duh, the files were stolen on the 19th, the files are dated on the 19th. Pretty easy trick. But think about this, the source, assets, & coconut monkey this guy stole & made a beta from has a whole lot of holes in it. You can see them in the screen shot, "ERROR", "ERROR", "ERROR", everywhere!! This is old material, not new data.
I'll re-iterate, slowly since you seem to have missed the point. Why do you believe the leaker was able to get hold of new source code, but only old game data? Do you honestly believe someone:
a) would steal the E3 material, but decide to hold onto it because they'll..
b) months later, also hack into valve
c) release the source
d) release the E3 material a few days later
no, if someone got hold of hl2 material at E3, it would have been released back then!
There's no doubt that a lot of this material is older (many of the files are named E3_whatever, they surely wouldn't have been worked on since), but this is no reason to believe that it's not the most up-to-date material valve had on the 19th when it was all stolen...